Catalytic Reactors

ABSTRACT

A compact catalytic reactor defining a multiplicity of alternately arranged first and second flow channels, for carrying first and second fluids, respectively. At least the first fluids undergo a chemical reaction. Each first flow channel contains a removable gas-permeable catalyst structure incorporating a metal foil substrate, said catalyst structure defining flow paths therethrough. The substrate comprises a stack of spaced-apart foils wherein the catalyst structure incorporates a multiplicity of resilient strips which are bent out from the foil substrate so as to project from the substrate and to support said catalyst structure resiliently spaced away from at least one adjacent wall of said channel. Each strip being connected to said catalyst structure only at an end or ends of said strip, and being integral with said foil.

This invention relates to a catalytic reactor suitable for use in achemical process to convert natural gas to longer-chain hydrocarbons,and to a plant including such catalytic reactors to perform the process.

A process is described in WO 01/51194 and WO 03/033131 (Accentus plc) inwhich methane is reacted with steam, to generate carbon monoxide andhydrogen in a first catalytic reactor; the resulting gas mixture is thenused to perform Fischer-Tropsch synthesis in a second catalytic reactor.The overall result is to convert methane to longer chain hydrocarbons ofhigher molecular weight, which are usually liquids or waxes underambient conditions. The two stages of the process, steam/methanereforming and Fischer-Tropsch synthesis, require different catalysts,and catalytic reactors are described for each stage. In each case thecatalyst may comprise a corrugated foil coated with catalytic material.In each case, the corrugated foils are of height substantially equal tothat of the channels: for example the channels might be of width 20 mmand depth 2.5 mm, the foil having corrugations 2.5 mm high.

According to the present invention there is provided a compact catalyticreactor defining a multiplicity of first and second flow channelsarranged alternately in the reactor, for carrying first and secondfluids, respectively, wherein at least the first fluids undergo achemical reaction; each first flow channel containing a removablegas-permeable catalyst structure incorporating a metal substrate, thecatalyst structure defining a multiplicity of parallel flow pathstherethrough; wherein the catalyst structure incorporates a multiplicityof projecting resilient lugs which support the catalyst structure spacedaway from at least one adjacent wall of the channel.

Preferably each catalyst structure incorporates resilient lugsprojecting in opposite directions, so that the catalyst structure isspaced away from both opposed adjacent walls of the channel. Wherechemical reactions are to take place in both the first and second flowchannels, then the second flow channels would also contain a removablegas-permeable catalyst structure incorporating such projecting resilientlugs.

The reactor may be made of an aluminium alloy, stainless steel,high-nickel alloys, or other steel alloys, depending on the temperatureand pressure required for the reactions, and the nature of the fluids,both reactants and products. The catalyst structures do not providestrength to the reactor, so the reactor itself must be sufficientlystrong to resist any pressure forces during operation. It will beappreciated that the reactor may be enclosed within a pressure vessel soas to reduce the pressure forces it experiences, or so that the pressureforces are only compressive.

The reactor must also be provided with headers to supply the fluids tothe flow channels, and preferably each first header comprises a chamberattached to the outside of the reactor and communicating with aplurality of the first flow channels, and each second header comprises achamber attached to the outside of the reactor and communicating with aplurality of the second flow channels, such that after removal ofa-header, the corresponding catalyst layers in the flow channels areremovable. This ensures that the catalysts can easily be replaced whenthey become spent.

The catalyst structure preferably incorporates a ceramic coating tocarry the catalytic material. Preferably the metal substrate for thecatalyst structure is a steel alloy that forms an adherent surfacecoating of aluminium oxide when heated, for example an aluminium-bearingferritic steel such as iron with 15% chromium, 4% aluminium, and 0.3%yttrium (e.g. Fecralloy (TM)). When this metal is heated in air it formsan adherent oxide coating of alumina, which protects the alloy againstfurther oxidation and against corrosion. Where the ceramic coating is ofalumina, this appears to bond to the oxide coating on the surface. Thesubstrate may be a wire mesh or a felt sheet, but the preferredsubstrate is a thin metal foil for example of thickness less than 100μm, and the substrate may be corrugated, pleated or otherwise shaped soas to define the multiplicity of flow paths.

Preferably the substrate of the catalyst structure is a foil corrugatedinto castellations (rectangular corrugations), and the resilient lugsproject above and below the castellations, being integral with the foiland being formed by punching out from the castellated foil. Other shapesof corrugations are also possible.

The metal substrate of the catalyst structure within the flow channelsenhances heat transfer within the catalyst structure, preventing hotspots or cold spots, enhances catalyst surface area, and providesmechanical strength. The projecting lugs ensure that the catalyststructure does not become jammed in the channel, for example due todifferential thermal expansion, and the lugs also allow for differencesin the dimensions of the catalyst structure and the channel that mayarise due to manufacturing tolerances. The lugs also allow all thesurfaces of the catalyst structure to be effectively contacted by theflowing reactants, as a gap is created between the channel walls and thecatalyst structure. The flow paths defined by the catalyst structure mayhave any suitable cross-sectional shape, but would typically berectangular; and by virtue of the gaps between projecting lugs adjacentflow paths along the outside of the catalyst structure communicate witheach other. Preferably all the surfaces forming the catalyst structureincorporate catalytic material.

Where the channel depth is no more than about 3 mm, then the catalyststructure may for example be a single shaped foil. Alternatively, andparticularly where the channel depth is greater than about 2 mm, thecatalyst structure may comprise a plurality of such shaped foilsseparated by substantially flat foils; the shaped foils and flat foilsmay be linked to each other, for example by similar projecting lugslocating in corresponding slots, or alternatively may be inserted asseparate items. To ensure the required good thermal contact, for examplewith a Fischer-Tropsch reactor, the channels are preferably less than 20mm deep, and more preferably less than 10 mm deep, and for asteam/methane reforming reactor the channels are preferably less than 5mm deep. But the channels are preferably at least 1 mm deep, or itbecomes difficult to insert the catalyst structures, and engineeringtolerances become more critical. Desirably the temperature within thechannels is maintained uniformly across the channel width, within about2-4° C., and this is more difficult to achieve the larger the channelbecomes.

The reactor may comprise a stack of plates. For example, first andsecond flow channels may be defined by grooves in respective plates, theplates being stacked and then bonded together. Alternatively the flowchannels may be defined by thin metal sheets that are castellated andstacked alternately with flat sheets; the edges of the flow channels maybe defined by sealing strips. The stack of plates forming the reactor isbonded together for example by diffusion bonding, brazing, or hotisostatic pressing.

Hence a plant for processing natural gas to obtain longer chainhydrocarbons may incorporate a steam/methane reforming reactor of theinvention, to react methane with steam to form synthesis gas, and aFischer-Tropsch reactor of the invention to generate longer-chainhydrocarbons.

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawings, inwhich:

FIG. 1 shows a sectional view of part of a compact catalytic reactor;

FIG. 2 shows a catalyst carrier for use in the reactor of FIG. 1;

FIG. 3 shows a sectional view of the catalyst carrier of FIG. 2, on theline 3-3 of FIG. 2.

The invention is applicable to a wide range of different chemicalreactions, particularly those involving gaseous reactants and requiringa catalyst. For example it would be applicable in a chemical process forconverting natural gas (primarily methane) to longer chain hydrocarbons.This can be achieved by a two-stage process, and each stage might use areactor of the invention. The first stage is steam reforming, in whichsteam is mixed with natural gas and heated to an elevated temperature(so as to reach say 800° C.) so that reforming occurs:

H₂O+CH₄→CO+3 H₂

This reaction is endothermic, and may be catalysed by a rhodium orplatinum/rhodium catalyst in a flow channel. The heat required to causethis reaction may be provided by combustion of an inflammable gas suchas methane or hydrogen, which is exothermic and may be catalysed bya-platinum/palladium catalyst in an adjacent second gas flow channel.

The gas mixture produced by the steam/methane reforming is then used toperform a Fischer-Tropsch synthesis to generate a longer chainhydrocarbon, that is to say:

n CO+2n H₂→(CH₂)_(n)+H₂O

which is an exothermic reaction, occurring at an elevated temperature,typically between 190° C. and 280° C., and an elevated pressuretypically between 1.5 MPa and 2.5 MPa (absolute values), in the presenceof a catalyst such as iron, cobalt or fused magnetite. The preferredcatalyst for the Fischer-Tropsch synthesis comprises a coating ofgamma-alumina of specific surface area 140-230 m²/g with about 10-40%cobalt (by weight compared to the alumina), and with a promoter such asruthenium, platinum or gadolinium which is less than 10% the weight ofthe cobalt, and a basicity promoter such as lanthanum oxide.

The stream of high pressure carbon monoxide and hydrogen produced bysteam methane reforming is cooled and compressed to the elevatedpressure, say 2.0 MPa, and is then fed to a catalytic Fischer-Tropschreactor, which may be a reactor of the invention; the reactant mixtureflows through one set of channels, while a coolant flows through theother set.

The reaction products from the Fischer-Tropsch synthesis, predominantlywater and hydrocarbons such as paraffins, are cooled to condense theliquids by passage through a heat exchanger and a cyclone separatorfollowed by a separating chamber in which the three phases water,hydrocarbons and tail gases separate, and the hydrocarbon product isstabilised at atmospheric pressure. The hydrocarbons that remain in thegas phase and excess hydrogen gas (the Fischer-Tropsch tail gases) arecollected and split. A proportion may be passed through a pressurereduction valve to provide the fuel for the catalytic combustion processin the reformer (as described above). The remaining tail gases may befed to a gas turbine arranged to generate electricity. The major plantelectrical power needs are the compressors used to raise the pressure tothat required for the Fischer-Tropsch reaction; electricity may also beused to operate a vacuum distillation unit to provide process water forsteam generation.

Referring now to FIG. 1 there is shown a reactor block 10 suitable foruse as a steam reforming reactor, with the components separated forclarity. The reactor block 10 consists of a stack of plates that arerectangular in plan view, each plate being of corrosion resistanthigh-temperature steel such as Inconel 800HT or Haynes HR-120. Flatplates 12 of thickness 1 mm are arranged alternately with castellatedplates 14, 15 in which the castellations are such as to definestraight-through channels 16, 17 from one side of the plate to theother. The castellated plates 14 and 15 are arranged in the stackalternately, so the channels 16, 17 are oriented in orthogonaldirections in alternate castellated plates 14, 15. The thickness of thecastellated plates 14 and 15 (typically in the range between 0.2 and 3.5mm) is in each case 0.75 mm. The height of the castellations (typicallyin the range 2-10 mm) is 3 mm in this example, and solid edge strips 18of the same thickness are provided along the sides. In the castellatedplates 15 which define the combustion channels 17 the wavelength of thecastellations is such that successive ligaments are 25 mm apart, whilein the castellated plates 14 which define the reforming channels 16successive ligaments are 15 mm apart.

A reactor block similar to that of FIG. 1 would also be suitable for useas a Fischer-Tropsch reactor, in this case defining channels for acoolant fluid alternating with channels for the Fischer-Tropschsynthesis. The channels for coolant might for example be 2 mm high(typically in the range 1 to 4 mm) and channels for the Fischer-Tropschsynthesis might be of height 5 mm (typically in the range 3 to 10 mm).In this case the reactor does not operate at such a high-temperature, sothe structural components may be of aluminium alloy, for example 3003grade (aluminium with about 1.2% manganese and 0.1% copper).

In either case the stack is assembled as described above, and bondedtogether, for example by brazing or hot isostatic pressing. Catalystcarriers 20 (only two are shown) are then inserted into the channels inwhich reactions are to occur, carrying appropriate catalysts.Appropriate headers can then be attached to the outside of the stack.Each catalyst carrier 20 incorporates a metal foil substrate whichassists in dissipating heat uniformly across the surface of the catalystto reduce or eliminate the development of hot spots, and also providesstructural integrity to the catalyst. However it has now beenappreciated that in many cases heat conduction from the catalyst to thechannel walls is not critical; in the case of the combustion andreforming reactions, this is because heat transfer between the catalystcarrier 20 and the walls of the channel occurs primarily throughconvection and radiation; and in the case of the Fischer-Tropschreaction the bulk of the heat transfer occurs through convection betweenthe catalyst carrier 20 and the walls of the channel.

Referring now to FIG. 2, in which the channel walls are shown only bybroken lines, each catalyst carrier 20 comprises a 50 μm thick Fecralloyfoil corrugated into a castellated shape (with rectangularcorrugations), the total height of the corrugations being 1 mm less thanthe height of the channel. The foil is coated with a ceramic coating(not shown separately in the figures) such as alumina of thicknesstypically in the range 30-80 μm (for combustion or steam reforming), andthe active catalytic material (such as platinum/rhodium, in the case ofsteam reforming) is incorporated into the ceramic. Resilient lugs 22 arealso stamped out from the foil during the corrugation process, thesebeing for example of width 0.5 mm and length 1.5 mm, remaining integralwith the foil at one end, and projecting above or below thecorrugations. These may for example be provided at 25 mm spacings alongthe length of the foil, and may (as shown) be provided in everycorrugation, or at any rate at least once every two or threecorrugations across the width of the foil.

As shown in FIG. 3, the lugs 22 projecting above and below the catalystcarrier 20 are at substantially the same positions along the length ofthe catalyst carrier 20, and when the catalyst carrier 20 is insertedinto the channel the lugs 22 are compressed and pushed into a slopingposition. The catalyst carrier 20 is therefore supported resiliently bythe lugs 22.

It will be appreciated that the resilience of the lugs 22 canaccommodate for any differential thermal expansion of the reactor blockand the catalyst carriers 20, and for any bowing of the channel as aresult of thermal stress, and also allows for any discrepancy betweenthe height of the catalyst carrier 20 and that of the correspondingchannel (as can arise as a result of manufacturing tolerances). However,the lugs 22 require that the catalyst carriers 20 are both installed andremoved in the direction of the arrow A in FIG. 3, so that removableheaders must be provided at both ends of each channel.

It will be appreciated that the corrugations might have a differentshape to that shown here; they may for example have a different ratio ofheight to width of each corrugation from that shown, for example beingsquare rather than rectangular corrugations. Furthermore thecorrugations might be a different shape altogether, for example thevertical parts of the corrugations shown in FIG. 2 might instead beinclined to form a zigzag shape with flat tops; yet again thecorrugations might instead be arcuate or sinusoidal. The lugs might bespaced differently from those described above and might be of adifferent shape or size. There might be lugs on only one side of thecatalyst insert, instead of lugs being provided on both the opposedsurfaces.

It will also be appreciated that in some cases, for example with achannel that is of height above about 4 mm, it is appropriate to use anassembly of corrugated catalyst carriers separated by substantially flatfoils which may also be catalyst carriers. For example in a 6 mm channelthere might be two corrugated foil catalyst carriers each of height 2.5mm, separated by a substantially flat foil. In this case lugs need onlybe provided on the surfaces of catalyst carriers that are adjacent to awall of the channel—projecting from the top of the top corrugated foil,and from the bottom of the bottom corrugated foil, in this example.Similar lugs might also be provided for interlocking the foils together,for example lugs projecting from the lower surface of the uppercorrugated catalyst carrier and from the upper surface of the lowercorrugated catalyst carrier might locate in corresponding slots in theflat foil; such lugs might be inclined in the opposite direction tothose adjacent to the channel walls. And alternatively lugs might beprovided projecting below and above the flat foil, and locating incorresponding slots in the corrugated foils.

It will be appreciated that this catalyst structure, because it isspaced apart from the wall by the lugs 22, provides an increasedcross-sectional area for fluid flow, reducing the local gas velocity andthe pressure drop across the reactor block.

1-10. (canceled)
 11. A compact catalytic reactor defining a multiplicityof first and second flow channels arranged alternately in said reactor,for carrying first and second fluids, respectively, wherein at leastsaid first fluids undergo a chemical reaction; wherein each first flowchannel contains a removable gas-permeable catalyst structureincorporating a metal foil substrate, said catalyst structure definingflow paths therethrough; wherein said substrate comprises a stack ofspaced-apart foils; wherein said catalyst structure incorporates amultiplicity of resilient strips which are bent out from the foilsubstrate so as to project from the substrate and to support saidcatalyst structure resiliently spaced away from at least one adjacentwall of said channel, each strip being connected to said catalyststructure only at an end or ends of said strip, and being integral withsaid foil; and wherein said catalyst structure, excluding saidprojecting strips, is of height less than a corresponding height of saidchannel by between 0.1 mm and about 1 mm.
 12. A catalytic reactor asclaimed in claim 11 wherein chemical reactions occur in both said firstand said second flow channels, and wherein second flow channels containremovable gas-permeable catalyst structures that incorporate a metalfoil substrate, and which define flow paths therethrough, wherein saidcatalyst structure incorporates a multiplicity of projecting resilientstrips which support said catalyst structure spaced away from at leastone adjacent wall of said channel, each strip being connected to saidcatalyst structure only at ends of said strip, and being integral withsaid foil.
 13. A catalytic reactor as claimed in claim 11 wherein saidcatalyst structure comprises resilient strips projecting in oppositedirections, so that said catalyst structure is spaced away from bothopposed adjacent walls of said channel.
 14. A catalytic reactor asclaimed in claim 11 wherein said resilient strips comprise projectinglugs, attached to said foil at one end.
 15. A catalytic reactor asclaimed in claim 11 wherein said resilient strips comprise projectingcurves, each being attached to said foil at both ends.
 16. A catalyticreactor as claimed in claim 11 wherein the said substrate comprises astack of spaced-apart foils in which at least some of the foils in thestack are spaced apart by resilient strips which are bent out from thefoil substrate so as to project from the substrate.
 17. A catalyticreactor as claimed in claim 11 wherein said first fluids undergo steamreforming.
 18. A catalytic reactor as claimed in claim 11 wherein saidfirst fluids undergo Fischer-Tropsch synthesis.
 19. A plant forprocessing natural gas for obtaining longer chain hydrocarbons, saidplant comprising: a first reactor as defined by claim 11, wherein saidfirst fluids undergo steam reforming, for reacting methane with steamfor forming synthesis gas, and a second reactor as defined by claim 11,wherein the second reactor is configured to be fed a stream from thefirst reactor; wherein said first fluids undergo Fischer-Tropschsynthesis for generating longer-chain hydrocarbons; wherein the firstreactor is a reforming reactor; and wherein the second reactor is asynthesis reactor.
 20. A compact catalytic reactor defining amultiplicity of first and second flow channels arranged alternately inthe reactor, for carrying first and second fluids, respectively, whereinat least said first fluids undergo a chemical reaction; wherein eachfirst flow channel contains a removable gas-permeable catalyst structureincorporating a metal foil substrate, said catalyst structure definingflow paths therethrough; wherein said substrate comprises a stack ofspaced-apart foils; wherein said catalyst structure incorporates amultiplicity of resilient strips which are bent out from the foilsubstrate so as to project from the substrate and to support saidcatalyst structure resiliently spaced away from at least one adjacentwall of said channel, each strip being connected to the catalyststructure only at an end or ends of the strip, and being integral with asaid foil.
 21. A compact catalytic reactor as claimed in claim 20wherein the said substrate comprises a stack of spaced-apart foils inwhich at least some of the foils in the stack are spaced apart byresilient strips which are bent out from the foil substrate so as toproject from the substrate.
 22. A compact catalytic reactor as claimedin claim 21 wherein adjacent foils in the stack are spaced apart byresilient strips which are bent out from each adjacent foil, whereinsaid adjacent foils are secured to each other by means that interlocksaid resilient strips bent out from the adjacent foils.